A preliminary investigation shows that a blast at a coal mine in Shanxi province that claimed the lives of 15 people resulted from illegal exploitation, the country's top coal mine safety watchdog said on Tuesday.

The explosion happened at around 1:50 pm on Monday while 34 miners were working underground in Pingyao county. Nineteen people escaped, among whom nine were injured. The other 15 miners were trapped and later confirmed dead, according to local authorities.

The accident was due to the exploitation in pillar areas that should have been left untouched, the National Coal Mine Safety Administration said in a release on Tuesday.

In coal mines, it is common practice to have such pillar areas to support the mine roof and prevent collapse. However, the mine in Shanxi used explosives to exploit the areas, according to the administration.

The damage to the pillar areas resulted in a large overflow of methane gas, which exploded upon meeting a spark, it said.

"The accident shows that leading executives of the coal mine were blinded by greed, and they crossed red lines by illegally exploiting pillars in different working areas," the administration said.

In addition, they illegally subcontracted mine sections, resulting in intensive exploitation at different sections at the same time. The coal mine, which had a mediocre ventilation system, also failed to install surveillance cameras as required, according to the administration.

Further, the positioning facilities of the mine did not work, and its map does not show the internal structures of the mine accordingly, it said.

"This was a production safety accident caused by lawbreaking production activities. It reveals problems with the company, such as a lack of awareness of the rule of law and chaotic management," Shen Xuping, director of the accident investigation division of the Shanxi Administration of Coal Mine Safety, said at a news conference on Tuesday morning.

Wang Guimei, director of Pingyao's health bureau, said the nine injured miners, who mainly suffered from trauma and burns, are in stable condition.

The county government has dispatched 15 work teams to help deal with the aftermath and compensation for the families of the 15 victims, according to local authorities.

Xinhua contributed to this story.